Field of Invention
This invention relates to a hydrocyclone. More specifically, it is directed to a high efficiency hydrocyclone for separation of a lower density liquid from a higher density liquid that utilizes a separation chamber that continually and gradually tapers.
The device is a hydraulically efficient hydrocyclone for separating a fluid mixture with a lighter phase liquid dispersed into a heavier phase liquid. The device is of particular importance in separating relatively low concentrations of lighter liquid from a continuous phase heavier liquid. The separator comprises a separation section with a gradual decrease in cross-sectional area throughout the entire length of the hydrocyclone. The hydrocyclone comprises an inlet area at the major diameter of the separation section and at least one outlet at the smaller diameter of the taper and an overflow outlet located centrally at the major diameter end of the taper.
The present invention relates to the separation of two immiscible liquids of differing densities through the efficient use of centrifugal force generated in a non-rotating chamber. The device has particular relevance to the oil and gas industries where large volumes of oil and water must be separated.
Hydrocyclones have been in use for many years dating back to the 1800's. Hydrocyclones have been adapted for use in separation of solids from liquid, solids from gas and gas from liquid. More recently, hydrocyclones have been applied to the separation of liquids from other liquids.
Studies conducted by Bradley in 1950's dealt primarily with conventional practices of hydrocyclones in solid--liquid applications. Bradley conducted limited studies of the use of hydrocyclones for separation of two liquid phases. He found that the separation of two liquids was much more difficult. This was due in part to the fact that liquid droplets dispersed in another fluid are much more fragile especially in the high shear. environment of hydrocyclones. The high shear stresses fragment the droplets into even smaller particles making them more difficult to separate. Separation is further hindered by the relatively low density differences between the fluids as compared to separation of solids or gasses which have relatively high density differences.
Later work by Listewnick and Regher also attempted to apply cyclones to liquid--liquid separation but had only limited success.
In the late 1970's, Professor Martin Thew conducted an in depth study of liquid--liquid separation using hydrocyclones. He found that using a series of cylindrical and tapering sections in combination substantially improved separation. These findings are described in U.S. Pat. No. 4,237,006, Colman et al. and further refined in U.S. Pat. Nos. 4,576,724 and 4,722,796 by Derek Colman and Martin Thew.
Further studies in the mid 1980's showed that the use of curved wall geometries as described in U.S. Pat. No. 4,949,107 could further increase separation efficiency by reducing shear and flow stagnation. The geometries described in this patent suggests the use of an exponential equation to define the shape of the separation chamber. The particular shapes mentioned allow for a rapid reduction in cross section of the separating chamber at the inlet end of the cyclone.
Work was also conducted in the use of complex involute entry paths to the hydrocyclone in order to provide a more gradual inlet to the hydrocyclone. Although this technique was well known in its application of solid--liquid hydrocyclones, it was further studied and patented by Prendergast for liquid--liquid hydrocyclones in U.S. Pat. No. 4,710,299.
Additionally, U.S. Pat. No. 4,764,287 by Colman and Thew describe the use of a single inlet in the form of "an inward spiralling feed channel which may be involute in form." Though a single inlet is functional, using only a single inlet contributes to an unstable helical flow path within the vortex. Two or more evenly spaced entries provide a more stable central core.
Additional study by Kalnins and Mai in U.S. Pat. No. 5,071,556 indicates that smaller diameter hydrocyclones with a more rapid acceleration of the fluids in the inlet section followed by a longer separation section could provide the best overall efficiency. It is believed that the improvement in efficiency obtained by this work was more a function of the reduction in hydrocyclone diameter than by the shorter inlet accelerating section. Further, it is well known by those skilled in the art that a smaller diameter provides better separation due to the greater centrifugal forces obtained and due to the shorter distances that the particles must travel across the cyclone diameter. This effect is also well known from the classification of solids separation using hydrocyclones where a smaller diameter provides a lower cut size of particles.
In all of the above described references, the inventions describe hydrocyclones of complex geometric shapes as well as multiple taper and curved sections to achieve the desired performance. These complex shapes are costly to manufacture and require exact measurements.
More importantly, these complex shapes and transition points create several significant problems. First, the transition between various angles creates instability of the flow field and an increase in the energy loss of the flow. Secondly, the sharp reduction in area creates a much greater energy loss due to higher differential velocities between the fluid layers. Third, the rapid acceleration of fluids can cause Very high internal shear stresses leading to droplet breakup. These concepts are described in more detail below.
Fluid is introduced tangentially into the hydrocyclone at a relatively high velocity. The higher velocity outer fluid layers contact the lower velocity inner fluid layers and exert a torque thereon causing the inner fluid layers to rotate. Thus, the outer fluid layers have a relatively higher velocity than the inner fluid layers. However, the velocity differentials between the fluid layers cause energy loss in the flow field. This energy loss is caused, in part, by the transfer of fluid between fluid layers (i.e. radial flow of fluid), localized turbulent flow created by the velocity differentials, as well as other velocity transfer inefficiencies between the fluid layers.
The energy loss in the flow field is exacerbated at the angular transition areas between different taper sections. The greater the change in taper angle between the taper sections, the greater the change in rotational velocity of the fluid. In this area of velocity transition, the increased turbulence in the flow field and the increased transfer of fluid between fluid layers increases the energy loss in the fluid and creates a wave in the fluid core making removal of the core difficult and, thereby, reduces the capacity of the hydrocyclone. This wave in the core effects the minimum overflow flow rate and increases the minimum inlet flow rate required for efficient operation.
In addition, the flow in the hydrocyclone is subject to a helical precession effect. Precession is a complex motion executed by a rotating body subjected to a torque tending to change its axis of rotation. In a hydrocyclone, as the fluid enters tangentially, it applies a torque to the fluid in the hydrocyclone and creates a spiral, or cork screw, flow following a helical path. The spiral flow and the change in the axis of rotation produces an oscillating effect on the central core of lower density liquid. Helical precession intensifies in areas of transition from one taper angle to another taper angle. In these areas of elevated precession, the oscillation amplitude of the central core is magnified and creates an increased wave effect in the fluid core making removal of the core difficult and, thereby, reduces the capacity of the hydrocyclone. Detrimental helical precession is also seen and further intensified in hydrocyclones having a single inlet.
Reducing the area through which a fluid passes increases the energy loss of those fluids due, in part, to acceleration of the fluid. This acceleration results in higher velocity differentials between fluid layers. The high velocity of the inlet fluid is achieved by providing a relatively high pressure, high velocity fluid in the inlet fluid supply line. However, as the fluid in the hydrocyclone loses energy, it also loses velocity. Because the hydrocyclone depends upon centrifugal force for its operation, maintenance of a relatively high fluid velocity is crucial.
Often, hydrocyclones utilize rapid transitions in cross sectional area to accelerate the fluids. This type of fluid acceleration, however, requires much higher inlet pressures to maintain a given flow rate due to the increased energy loss. Because the inlet fluid necessarily enters the hydrocyclone at a higher relative velocity than the fluid in the hydrocyclone, providing higher inlet pressure and velocity also creates a greater loss of fluid energy at the inlet of the hydrocyclone.
Centrifugal force is a function of velocity and radius of the flow wherein:
Centrifugal Force .alpha.(Velocity).sup.2 /Radius
Therefore, the higher the velocity or the smaller the radius, the higher the centrifugal forces. Typical hydrocyclones use steep taper angles to accelerate the flow at the inlet end of the hydrocyclone. This generates high velocity at the inlet, but increases the energy loss of the fluid. The fluid energy loss leads to a loss in fluid rotational velocity at the outlet end of the hydrocyclone. Thus, at the smaller diameter outlet end of the hydrocyclone, the centrifugal forces are greatly reduced. Maintaining fluid velocity and, thus, centrifugal force, at the underflow end of the hydrocyclone is of critical importance because only in this area of the hydrocyclone can the smaller liquid droplets be separated. These smaller droplets require strong centrifugal forces and short migration distances in order to effectively separate from the higher density liquid.
Generally, liquid--liquid separating hydrocyclones include a cylindrical portion at the underflow end. The cylindrical portion provides an increased retention time to allow additional time for the lower density liquid droplets to migrate to the core. Inclusion of the cylindrical portion facilitates greater separation. However, because this portion of the hydrocyclone is cylindrical, the fluid loses velocity and centrifugal force. Therefore, the amount of increased separation due to the additional retention time is relatively small; and the separation in the cylindrical portion is relatively inefficient.
Lastly, the rapid acceleration of the fluids created by reduction in area as described in the prior art can create area of high shear stress. As the fluid experiences turbulence and shear stress, dispersed lower density liquid droplets that encounter these shear zones are literally torn apart creating many smaller droplets. The smaller droplets are more difficult to separate as they are less buoyant than larger droplets due to the smaller volume each small droplet occupies.
2. Related Art
Hydrocyclones have long been known in the prior art and the prolixity of prior references is widespread. Although solid--liquid hydrocyclones are shown in prior patents as early as the nineteenth century, hydrocyclones were not functionally applied to liquid--liquid separation until the late 1970's.
Illustrative of such liquid--liquid hydrocyclones are U.S. Pat. Nos. 4,237,006, 4,576,724, 4,722,796, 5,071,556, and 4,964,994.
Though prior liquid--liquid hydrocyclones are helpful in separating a lower density liquid from a higher density liquid, they can be improved to provide less fluid energy loss and higher capacity separation at a lower cost of manufacturing the hydrocyclone.